Have a Super Mario Galaxy savefile and she will be unlocked after 50 races or get 1 star in all 8 Mirror Mode Cups

Funky Kong

Unlock 4 Expert Staff Ghost Data in Time Trials

Baby Luigi

Unlock 8 Expert Staff Ghost Data in Time Trials

Birdo

Play Time Trials on 16 different courses or win 250 Wi-Fi matches.

Toadette

Play Time Trials on all 32 different courses or win 1000 Wi-Fi matches.

Mii (Outfit B)

Unlock all 32 Expert Staff Ghost Data in Time Trials

Unlockable Karts

After meeting the requirements, some unlockables may require you to back out of your save file and reselect it. Note: In different countries, the karts have different names but the unlocking methods are the same.

Unlockable

How to Unlock

Aero Glider (large kart)

Attain at least 1 Star in all 150cc Retro Cups

Sprinter (Medium Kart)

Unlocked 24 Expert Staff Ghosts

Blue Falcon (small kart)

Win Mirror Lightning Cup

Cheep Charger (small kart)

Attain at least 1 Star Rank for all 50cc Retro Grand Prix Cups

Honeycoupe (large kart)

Win 150cc Lightning Cup

Piranha Prowler (large kart)

Win 50cc Special Cup

Royal Racer (medium kart)

Win 150cc Leaf Cup

Turbo Blooper (medium kart)

Win 50cc Leaf Cup

Tiny Titan (small kart)

Beat 50 opponents during Wi-Fi play or unlock one expert staff ghost

Unlockable Bikes

After meeting the requirements, some unlockables may require you to back out of your save file and reselect it. Note: In different countries, the bikes have different names but the unlocking method is the same.

Unlockable

How to Unlock

Quacker (small bike)

Win 150cc Star Cup

Bubble Bike (small bike)

Win Mirror Leaf Cup

Magikruiser (small bike)

Play Time Trials on 8 different courses

Zip Zip (medium bike)

Win 100cc Lightning Cup

Sneakster (medium bike)

Attain at least 1 Star Rank for all 100cc Wii Grand Prix Cups

Dolphin Dasher (medium bike)

Win Mirror Star Cup

Shooting Star (large bike)

Win 100cc Star Cup

Phantom (large bike)

Win Mirror Special Cup

Spear (large bike)

Unlock 12 Expert Staff Ghost Data in Time Trials

Alternate Title Screen and Ending Screen

Completing every single cup with a first place unlocks two new alternating Title Screens and one new Ending Photo.

Have a star by your nickname

Playing extraordinarily well in Grand Prix races can produce a Star Ranking upto 3 at the end results instead of the standard “C, B or A.”

Collecting these stars in all Grand Prix’s adds a Star Icon next to your name in WiFi Races and on the Results Screen for your opponents to see.

(Note: Say, if you get a Three star rating in every Grand Prix except one which you got a One star rating in. You would get a One starred nickname until you manage to upgrade the rank.)

Unlockable

How to Unlock

One starred nickname

Get a one star rating in every Grand Prix

Double starred nickname

Get a two star rating in every Grand Prix

Triple starred nickname

Get a three star rating in every Grand Prix.

Golden Wii Wheel

Use the wii wheel extensively in all the modes, when you go online a golden wii wheel will be next to your name.

Karts and Bikes versions of 50cc and 100cc

You can unlock Karts and Bikes versions of 50cc and 100cc(like 150cc, but slower) by completing all the cups in that CC.

To activate these versions, press the + button on the Speed Select screen.

Unlockable

How to Unlock

50cc(Karts and Bikes)

Place 1st, 2nd, or 3rd in all 50cc cups

100cc(Kart and Bikes)

Place 1st, 2nd, or 3rd in all 100cc cups

Mario Kart Wii Secrets

Quick Start

When the countdown starts wait till the 2 starts to disappear and then press and hold the 2 button. So the 2 must still be visible but disappearing.
This will give you a boost at the start of the race.

Boost when recovering from a fall

When you fall into something, such as water or falling from rainbow road, when you land, if you hit 2 at the moment you hit the ground, you get a temporary boost.

POW Block Immunity

When you see the POW block on your screen and its about to hit you, just do the function you would normally use for a trick to create a shockwave for youself to become immune to the spin out the POW block creates (Control panel on GCN controller, Shaking Wii wheel, etc.). This can be done on both bikes and carts. Even though you don’t loose speed or spin out, you still will loose the items you were carrying.

Change Mii Racer weight class

Go into the Mii Channel and change your Mii drivers weight, which will have an in game impact causing you to change weight class and have different cars.

Mario Kart Wii Glitches

More than one person can be the same character.

First, make P1 choose the character you both want to be. Then unselect the character and move off of it. Move P2 over, and select the character. Go back one screen, then go into the character select again. You should both be on the same character. Both of you select the character. Do whatever you want from there on.

Note: In the character select screen and options, whenever one of you does an animation, both of you will. Also, if P2 chooses a different kind of vehicle than P1, P1 will look strange. Don’t worry, this won’t actually happen in the game.

I haven’t tested this with P3 or P4 yet, but in theory, it should work.